Modern warfare poses many risks to the men and women in war zones, including the men and women in the United States Marine Corps. One problem that Marines have in many conflicts is with untrained insurgents shooting at them from short range with assault rifles. These shooters are not trained snipers, but just lone individuals taking potshots at the Marines. Reacting to these shots can require the use of a Marine's time and resources that may better be used elsewhere. More significantly, often these shots hit their targets, injuring or even killing Marines in the line of fire. In many cases, the Marines cannot effectively respond to the shots or capture the shooter because they cannot accurately determine the direction from which the rifle was fired.
Often these shots come from assault rifles firing supersonic bullets. When a bullet is fired from a gun at supersonic speeds, the gunshot creates two sources of sound. The first sound is carried by an acoustic shock wave, akin to a small “sonic boom,” that is created by the bullet as it travels through the air at supersonic speeds. The second sound is the muzzle blast caused by the explosion of superheated gases from the muzzle of the gun.
Because the acoustic shock wave is caused by the bullet as it travels at supersonic speed, the first sound heard by a Marine in the area is caused by the acoustic shock wave, which travels outward in a cone from the bullet's trajectory. However, this shock wave does not come from the direction in which the bullet was fired but instead comes at an angle from that direction, and therefore the shock wave can't reliably be used to determine the source of the gunfire. In addition, although a gunshot will produce a shock wave as it travels through the air which can be perceived by a nearby Marine, the same perceived shock wave can also be produced by a gunshot traveling in a second direction. This ambiguity in direction cannot be resolved using only the direct sound from the shock wave.
The Marine may also hear the muzzle blast a short time after hearing the shock wave. Under conditions the muzzle blast can be used along with the shock wave to locate the shooter. However, the sound from the shock wave can be loud enough to reduce the hearing sensitivity of Marines within earshot for a short time, and so a Marine may not hear the muzzle blast. In addition, at longer ranges, many echoes of the muzzle blast are produced in an urban environment, making it impossible to decipher the muzzle blast direction. Moreover, if the Marine does not have a direct line of sight to the shooter, he may not hear the muzzle blast at all, even if his hearing sensitivity is not reduced by the shock wave.
Because of these shortcomings in a Marine's ability to determine the direction of incoming gunfire by simple auditory sensing, many developers are working on acoustic sniper detectors. Most of these detectors use arrays of sensors to detect both the shock wave and the muzzle blast and can give a very precise location for the sniper. In current use, these sensors are placed on poles, for example for ground- or vehicle-mounted sensor arrays, or are placed in arrays mounted on a Marine's helmet. However, none of these existing systems are compact enough to be carried by a single Marine without networking.
For example, in existing systems, achievement of high precision in determining the location of a shooter requires that the sensors be spread out over a large area. This can be achieved in many ways.
One current system is the Boomerang system produced by BBN Technologies of Cambridge, Mass. The Boomerang system comprises a vehicle-mounted array of microphones to detect incoming fire. However, the Boomerang system requires a computer such as a small desktop computer, and can take some time to respond, during which time many additional shots can be fired, posing additional risks to nearby Marines.
Other systems, such as ShotSpotter® system deployed in some locations use sensors mounted in multiple arrays connected by a wireless network between sensors mounted in multiple places. Such wireless networks can create problems with communications security and interference, and in the case of helmet-mounted sensors, can increase the risk to the Marines, with each Marine becoming a radio beacon broadcasting his location to anyone listening.
Thus, none of the existing sniper detection systems have proven to be satisfactory.